For certain towed vehicles, such as trailers rated at or above a certain minimum size, applicable laws and standards often require that the towed vehicle have an onboard system for actuating its braking system independently of the towing vehicle in the event that the towed vehicle become disengaged from the towing vehicle. Such towed vehicles typically include an electrically actuated brake system which is powered from the towing vehicle electrical system during normal towing conditions. If and when the towed vehicle breaks away from the towing vehicle, a secondary battery mounted on the towed vehicle supplies power to the electric brakes to bring the towed vehicle to a stop. However, before this ever happens, the secondary battery may become discharged and, therefore, may not be able to independently provide sufficient power to actuate the electric brakes on the towed vehicle. Because reliable brake actuation requires a fully charged battery, various systems have previously been developed to charge the battery on the towed object from the towing vehicle while the two are connected together, to thereby ensure that the secondary battery is sufficiently charged for such emergency conditions.
One such previously developed charging system is disclosed in U.S. Pat. No. 5,289,103, commonly owned with the present invention and expressly incorporated herein by reference, which pertains to a particularly advantageous battery charger for a towed vehicle. The battery charger disclosed in the '103 patent is a three-stage charger for recharging secondary batteries, typically sealed lead-acid batteries used in systems such as the above-described breakaway system. The charger of the '103 patent is temperature regulated, having a regulator that controls the charging voltage applied to the battery, such that the charger outputs a first level when the battery is charging at a constant current, a second level (comprising a constant voltage) when the desired final charge state is reached, and a third level comprising a "float" voltage for long-term storage. The charger of the '103 patent outputs a charging voltage at a controlled level regardless of whether the supply voltage is greater or less than the predetermined controlled charging level. This charger also includes a current-control driver circuit which controls the energy applied to the battery so that it receives an optimum level, to provide faster charging when the energy from the towing vehicle electrical system is greater. Because of its multiple capabilities and attributes, the system of the '103 patent is relatively complex and somewhat expensive to manufacture in relation to certain less expensive (and less capable) alternatives which have also been developed heretofore.
One very inexpensive commercially available charging device for batteries such as those mounted on a towed vehicle simply comprises a diode (often rated at three amperes of current) that is connected in series with a high-wattage resistor, typically having a resistance value within the range of 200 to 235 ohms. These components are electrically connected in series between the charging source, which is typically the positive terminal of the battery of the towing vehicle, and the positive terminal of the battery on the towed vehicle which needs to be charged, usually a lead-acid battery. During the beginning of charging operation, when the towed vehicle battery is very depleted (discharged) and the voltage differential between the charging battery and the battery to be charged is relatively large, the diode will conduct and the towing vehicle electrical system will begin to charge the towed vehicle battery. In this system, a high-wattage resistor is required because the current generated in the charging circuit may be large enough to result in a hazardous condition (e.g., excess heat, possibly causing a fire), particularly when the towing vehicle battery and the towed vehicle battery are improperly connected in reverse polarity, which creates a 24-volt potential, or when the battery on the towed vehicle is or becomes internally short-circuited. Although use of a high-wattage current-limiting resistor protects against such a potentially hazardous condition, a major drawback of this system is that, by including a relatively high resistance in the circuit, the normal or typical charging current is so small that the secondary battery receives very little charge, even over long periods of time. For example, if the towed vehicle battery is discharged to 12-volts and the charging (tow vehicle) battery is at 14-volts, there is a 2-volt differential across the diode/resistor combination (.about.200 ohms), which will result in a charge current of less than 10-milliamperes. At this rate, over five days of continuous charging, the battery which needs to be charged will not regain more than 0.1-volts of charge, thus making this system, at most, very marginally useful.
Therefore, a need exists for a battery charger for towed vehicle batteries that is relatively inexpensive and also capable of providing enough charge to insure that the secondary (towed vehicle) battery remains sufficiently charged.